1. Field of the Invention
The present invention relates to heating apparatuses and more particularly to heating apparatuses for heating packaging materials.
2. Description of the Prior Art
Packages or cartons of the single-use disposable type which hold liquid contents such as milk, juice or the like are often manufactured by reforming and sealing laminated packaging material. The packaging material consists wholly or partly of thermoplastic material which, during the reforming process into cartons, is heated to its softening or melting temperature point in order to make reforming and/or sealing of the material possible. One generally employed working operation is to heat the edge regions of a material web until the outer thermoplastic layer of the packaging laminate melts, whereafter the edge regions are compressed together and allowed to cool so that a liquid-tight seal is created.
The prior art heating apparatuses employed for carrying out the above-mentioned heating operation are of varying types, but operate remarkably often with hot air which is led to those surfaces of the material which are to be heated. Hot air heating is preferred in many cases, since a large number of prior art heating apparatuses are available on the market at a relatively reasonable cost, and such apparatuses share the common feature that hot air heating makes it possible to restrict the heating to a defined, optionally shaped region at the same time as the heating level may readily be regulated. Such heating apparatuses normally include an air nozzle which is aimed towards that region of the packaging material which is to be heated. Air is supplied to the nozzle by means of a fan that sucks in ambient air which, before reaching the nozzle, passes a heating device, normally some form of electric resistor element, so that the air, on departing from the nozzle, is at a desired temperature, generally between 200.degree. and 400.degree. C.
The disadvantages inherent in prior art heating apparatuses of the hot air type are primarily that they are extremely energy-intensive, since the heated air, after having come into contact with the packaging material and having given off but a minor fraction of its heat, flows out into the ambient atmosphere. This is not only a disadvantage from the energy viewpoint, but also entails an unnecessary and, in many cases, hazardous warming of the atmosphere. Another disadvantage which contributes to poor heat economy is the sluggishness of response shown by heating apparatuses of the resistor type to adjustment. In order to ensure that a sufficient volume of air at the desired temperature is available on those occasions when packaging materials are to be heated, the heating device proper normally operates continously and the generated hot air is channelled off into the atmosphere so that it does not impinge on the packaging material when this is not desirable, for instance between heating of successively advanced, sheet-formed packaging material blanks.
Thus, the low degree of efficiency of the prior art constructions is not only wasteful of energy, but also constitutes a hazardous warming of the atmosphere and it is, therefore, a general desire within this art to devise a heating apparatus of the hot air type which does not suffer from the above-mentioned drawbacks.